A DNA prime-modified vaccinia virus ankara boost vaccine encoding thrombospondin-related adhesion protein but not circumsporozoite protein partially protects healthy malaria-naive adults against Plasmodium falciparum sporozoite challenge

Abstract

The safety, immunogenicity, and efficacy of DNA and modified vaccinia virus Ankara (MVA) prime-boost regimes were assessed by using either thrombospondin-related adhesion protein (TRAP) with a multiple-epitope string ME (ME-TRAP) or the circumsporozoite protein (CS) of Plasmodium falciparum. Sixteen healthy subjects who never had malaria (malaria-naive subjects) received two priming vaccinations with DNA, followed by one boosting immunization with MVA, with either ME-TRAP or CS as the antigen. Immunogenicity was assessed by ex vivo gamma interferon (IFN-gamma) enzyme-linked immunospot assay (ELISPOT) and antibody assay. Two weeks after the final vaccination, the subjects underwent P. falciparum sporozoite challenge, with six unvaccinated controls. The vaccines were well tolerated and immunogenic, with the DDM-ME TRAP regimen producing stronger ex vivo IFN-gamma ELISPOT responses than DDM-CS. One of eight subjects receiving the DDM-ME TRAP regimen was completely protected against malaria challenge, with this group as a whole showing significant delay to parasitemia compared to controls (P = 0.045). The peak ex vivo IFN-gamma ELISPOT response in this group correlated strongly with the number of days to parasitemia (P = 0.033). No protection was observed in the DDM-CS group. Prime-boost vaccination with DNA and MVA encoding ME-TRAP but not CS resulted in partial protection against P. falciparum sporozoite challenge in the present study.

FIG. 1.

Geometric mean ex vivo ELISPOT responses to DDME-TRAP (A and B) and DDM-CS (C) for groups 1 and 2. Panel A shows the geometric mean summed ex vivo ELISPOT responses, in SFC per million PBMC, to TRAP for group 1 subjects receiving DDM-ME TRAP. D0 = day 0, D+7 = first DNA vaccination plus 7 days, D+28 = first DNA vaccination plus 28 days (i.e., the day of the second vaccination), DD+7 = second DNA vaccination + 7 days, DD+28 = second DNA vaccination plus 28 days (i.e., the day of the MVA vaccination), DDM+7 = MVA vaccination plus 7 days, DDMC = day of challenge (i.e., the day of the MVA vaccination plus 14 days), DDMC+7 = day of challenge plus 7 days, and DDMC+90 = day of challenge plus 90 days. Error bars are one standard error of the mean.

FIG. 2.

Cultured ex vivo ELISPOT responses. Cultured ELISPOT responses in SFC per million PBMC are shown for each subject at day 0, day of challenge (DOC; 14 days after final vaccination), and 90 days after challenge (C+90). Responses to TRAP (group 1 [A]) or to CS (group 2 [B]) were measured after a 9-day culture period. No data are shown for the following time points due the lack of samples for analysis: subject 392, C+90, subject 397, DOC and C+90; and subject 401, DOC. Error bars are one standard error of the mean.

FIG. 3.

Anti-TRAP and anti-CS antibody titers. The levels of anti-TRAP antibodies for group 1 subjects (A) and anti-CS repeat antibodies for group 2 subjects (B) are shown and are expressed as geometric mean titers relative to day 0. D0 = day 0, D+7 = first DNA vaccination plus 7 days, D+28 = first DNA vaccination plus 28 days (i.e., the day of the second vaccination), DD+7 = second DNA vaccination plus 7 days, DD+28 = second DNA vaccination plus 28 days (i.e., the day of the MVA vaccination), DDM+7 = MVA vaccination plus 7 days, DDMC = day of challenge (i.e., the day of the MVA vaccination plus 14 days), DDMC+7 = day of challenge plus 7 days, and DDMC+90 = day of challenge plus 90 days. Error bars are one standard error of the mean.

FIG. 4.

Kaplan-Meier survival curves postchallenge. The survival curves for each group are shown, with days to parasitemia representing the number of days after experimental P. falciparum challenge that each subject received a diagnosis of malaria. Six of six control subjects, seven of eight group 1 DDM-ME TRAP subjects, and eight of eight group 2 DDM-CS subjects were diagnosed with malaria. One group 1 subject, subject 405, did not develop malaria. There was a significant delay to parasitemia for the DDM-ME TRAP group compared to control subjects (P = 0.045 [log-rank test]) but not for the DDM-CS group and the controls.

FIG. 5.

Correlation between ex vivo IFN-γ ELISPOT responses at DDM+7 and days to parasitemia for group 1 (DDM-ME TRAP). The summed ex vivo IFN-γ ELISPOT response to ME TRAP 7 days after the final vaccination (DDM+7) for each group 1 subject correlates with the number of days to parasitemia as measured by Spearman's rank correlation coefficient (r = 0.793, two tailed, P = 0.033). One subject had been excluded from all immunogenicity analysis because of a very high background at one time point.